Various types of combustion equipment and absolutely essential for human life. For example, boilers installed in thermal powerstations supply electric power and various types of internal combustion engines are mounted on cars which are one of the major transportation means in modern society. Unfortunately, however, this combustion equipment is inevitably accompanied by the possibility of emission of a considerable quantity of injurious gases, depending on the condition in which the combustion takes place. Therefore, a long standing strong requirement is recognized for the development of combustion equipment which is free from the air contamination problem, in addition to the traditional requirement for development of combustion equipment having a higher combustion efficiency and which requires less quantity of fuel for generating a unit quantity of energy. Further, it is recognized that a new requirement has come out for development of a technology for cleaning the air contaminated with injurious gases.
It is believed that combustion of a lean fuel or combustion of a mixture of a less quantity of a gaseous fuel and a larger quantity of air is effective to simulaneously satisfy both requirements for a decreased emission of injurious gases and for an increased combustion efficiency of a combustion equipment. For example, it is publicly known that a lean fuel mixture (hereafter lean fuel) is preferably employed for a Diesel engine. Therefore, it is hopefully assumed that a lean fuel could be employed for a gasoline engine to satisfy the foregoing objects.
However, mixture containing a fuel and air at an undersired content ratio readily causes such engines to exhaust a considerable quantity of soot and/or causes misfiring, despite the fact that such engines inherently prefer a lean fuel, causing various problems including the air pollution problem due to the emission of soot or the unburned fuel and the less satisfactory magnitude of combustion efficiency. In other words, such a mis-use of a lean fuel is not only ineffective to satisfy the foregoing objects but also involved with a possibility that it causes various reverse effects. Therefore, adjustment of the ratio of fuel and air is an extremely important item to allow combustion equipment to operate under satisfactory conditions. It is quite true for any type of control system that the accurate and quick detection of an object to be controlled (in this case, the ratio of fuel and air in a range in which the fuel ratio is relatively small.) is essential for performance of the control system. Unfortunately, however, none of the sensors which are at present available in the prior art is satisfactory for the foregoing purposes. For example, the magnetic oxygen concentration detector is unsatisfactory for the purpose to be employed under a condition wherein the detector is mounted on a car, because of its rather slow response speed. The density type sensor or the thermal conductivity type sensor is also unsatisfactory for the purpose to control the combustion of an internal engine, because the accuracy thereof is inclined to be infuenced even by a marginal quantity of hydrogen (H.sub.2) contained in the gas.
The known equipment for detecting oxygen concentration are represented by a limiting electric current type oxygen concentration detector which was invented by the inventors of this invention and was laid open to public inspection under the Laying-open of Application No. Toku-Kai-Sho No. 52-72286 in Japan and which disclosed the conceptual construction of the limiting electric current type oxyten concentration detector and by another limiting electric current type oxygen concentration detector which was invented by the inventors of this invention and was filed under the application No. 55-123677 in Japan which disclosed an improvement applied to the foregoing conceptual construction or an improved construction of a limiting electric current type oxygen concentration detector of which the cathode is covered by a porous material.
Either of these limiting current type oxygen concentration detectors is free from various drawbacks which are inevitably involved with the oxygen concentration detector available in the prior art. From this viewpoint, either of these limiting current type oxygen concentration detector is recognized as an oxygen concentration detector having an excellent feature. From other view points, however, either of the foregoing limiting current type oxygen concentration detectors is involved with possibilities of further improvement. It is quite often than internal combustion engines vary the temperature of their exhaust gas depending on the corresponding operating conditions. Therefore, a limiting current type oxygen concentration detector is required to have a stable performance in a relatively wide range of oxygen concentration, when it is employed as a detector for an exhaust gas. However, a limiting current type oxygen concentration detector is inclined to vary the oxygen concentration vs. the limiting current relations depending on the temperature of the sensor even under a constant voltage applied thereto. As a result, the limiting current type oxygen concentration detector is involved with a considerable amount of error caused by variation of the temperature of a gas and the oxygen concentration range in which the reliable measurement is available, is fairly limited. Further, the limiting current type oxygen concentration detector has a tendency, under a constant oxygen concentration, to sharply decrease the amount of the limiting current following a decrease in temperature in a low temperature range, which reduces the temperature range in which reliable operation is allowed. This sharp decrease of the limiting current in a low temperature range is caused by an increase of the internal resistance of the oxygen sensor.